Tank or Tankless

I currently have a very inefficient water heating system that I m attempting to upgrade. I have two showers, dishwasher, and a washing machine. In lieu of any existing tank I get my hot water from the coil of my oil boiler (hot water). I have talked to various plumbers and half suggest and indirect water tank that is fed off of my boiler (with the possibility of hooking the heater into an existing gas line to be utilized in the summer). The other half are suggesting a tankless water heater. What is the most efficient system.

Using the wast heat from the boiler is great and can be uses with either a storage tank ie. standard water heater, or a tankless unit. Two showers and a dishwasher can be handled by either system, for the tankless will need to look at gas pipe size as well as flew size, though most ar direct vent now, using a blower and a 2" PVC exhaust line

How old and how efficient is the boiler? Older ones tend to be inefficient, but that is not always the case. Tankless size depends on peak needs and worst case incoming water temp. The more you need at once and the colder the incoming water, the bigger the thing needs to be (and that may require redoing your gas service - don't even think about trying this with electric). You might have to upgrade the gas service all the way back to the main line, and that could be expensive. Maintenance on a typical tank is close to zero (although you should probably drain the sediment out of it periodically, not that many people do). If you don't do regular maintenace on a tankless, you'll likely notice a degradation in performance fairly quickly. The newer boilers are designed to not maintain some minimum temp, so they typically only fire when there's a need. Older ones can't handle that. An indirect would approach your boiler's efficiency during the heating season, but if it can't cool off during the summer, the standby losses mount up. If you could throw drainwater heat recovery into the mix, tankless looks better, when used with a shower where the heated water is constantly going down the drain. This will help keep the size required down some.